Mediator Design Pattern (Behavioral) -Seema Joshi

Intent Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently. 03/04/03 CS590L Mediator Pattern

Motivation Object oriented design encourages the distribution of behavior among objects. Partitioning a system into many objects enhances reusability. Many connections between objects in an object structure. -Lots of interconnections increases interdependency. -Moreover it can be difficult to change the systems' behavior in any significant way, since behavior is distributed among many objects. 03/04/03 CS590L Mediator Pattern

Applicability When one or more objects must interact with several different objects. When simple object need to communicate in complex ways. When you want to reuse an object that frequently interacts with other objects. 03/04/03 CS590L Mediator Pattern

Example- Problem 03/04/03 CS590L Mediator Pattern

Example -Solution 03/04/03 CS590L Mediator Pattern

Example -Sequence Diagram 03/04/03 CS590L Mediator Pattern

Example -Steps The list box tells its director that it’s changed. The director gets the selection from the list box. The director passes the selection to the entry field. Director takes action. 03/04/03 CS590L Mediator Pattern

Example -Structure 03/04/03 CS590L Mediator Pattern

Example -Relationship DialogDirector is an abstract class that defines the overall behavior of a dialog. Clients call ShowDialog() CreateWidgets() –abstract operation for creating widgets. 03/04/03 CS590L Mediator Pattern

General Structure 03/04/03 CS590L Mediator Pattern

Participants Mediator ConcreteMediator Colleague classes -defines an interface for communicating with Colleague objects. ConcreteMediator -implements cooperative behavior by coordinating Colleague objects. -knows and maintains its colleagues. Colleague classes -each Colleague class knows its Mediator object. -each colleague communicates with its mediator whenever it would have otherwise communicated with another colleague. 03/04/03 CS590L Mediator Pattern

Another Example 03/04/03 CS590L Mediator Pattern

Good Points about the Mediator Centralized control Less chance of miscommunication. Simplifies object protocols Replaces many-to-many interactions with one-to-many interactions between mediator and colleagues, thus simple to understand, maintain and extend. Abstracts how objects cooperate Splitting up the system in this way creates a better understanding of the objects in the system, how they interact and how they are structured . Loose coupling between colleagues promotes Reusability Basically, it proliferates interconnections to help eventually reduce it. 03/04/03 CS590L Mediator Pattern

Bad Points about the Mediator Overloading the Mediator There is the potential for numerous subclasses of the Mediator and these in turn could require subclasses of those Mediator subclasses. It's just a horrible cycle from this point on. Hindrance If there are a relatively small group of objects, it may waste less time if they are able to speak to each other directly rather than through a Mediator. 03/04/03 CS590L Mediator Pattern

Related Patterns Facade Observer 03/04/03 CS590L Mediator Pattern

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References Design Patterns: Elements of Reuseable Object-Oriented Software by E. Gamma, R. Helm, R. Johnson and J. Vlissides, Addison-Wesley, ISBN: 0-201-63361-2, Copyright 1995. http://sern.ucalgary.ca/courses/SENG/443/W02/assignments/Mediator/ 03/04/03 CS590L Mediator Pattern

Thank You 03/04/03 CS590L Mediator Pattern